The invention relates to an exhaust gas turbocharger for an internal combustion engine including an exhaust gas turbine, a compressor and an electric motor.
Publication DE-OS 28 08 147 describes such an exhaust gas turbocharger which comprises an exhaust gas turbine acted on by the engine exhaust gases, and a compressor wheel in the intake section of the internal combustion engine, which wheel is driven by the exhaust gas turbine and via which combustion air taken in is compressed to an elevated boost pressure. Integrated into the exhaust gas turbocharger is an electric motor, the armature of which is located on a shaft between the turbine wheel and the compressor wheel and the stator of which is located in a stationary manner in the housing of the charger and surrounds the armature radially. By actuating the electric motor, energy is additionally supplied to the exhaust gas turbocharger, so that, in particular in operating states of the internal combustion engine in which the exhaust gas back pressure is not sufficient to generate a sufficiently high boost pressure, the compressor can be driven additionally so that a desired boost pressure level can be obtained.
The electric motor can also be used as a generator, when the energy obtained from the exhaust gas turbine is greater than the energy required in the compressor for generating the necessary boost pressure. Use as a motor and as a generator improves the overall engine efficiency.
The armature of the electric motor extends axially over the length of the shaft between the turbine and the compressor, so that the mass moment of inertia of the rotating parts of the exhaust gas turbocharger is distinctly greater in comparison with a version without an electric motor. The increase in the mass moment of inertia in turn reduces the efficiency of the charger.
Comparable exhaust gas turbochargers, which are combined with electric motors, are also described in publications EP 0 420 666 A1, U.S. Pat. No. 5,605,045 and U.S. Pat. No. 5,074,115.
A further version of an exhaust gas turbocharger driven by an electric motor is disclosed in US 5 870 894. According to this publication, the compressor wheel includes on its rear side facing the turbine, a radial recess, in which a magnet is disposed, which is connected in a rotationally fixed manner to the compressor wheel and interacts with coils which are fixed to the housing so that current can be supplied to the coils. In this way, a relatively compact construction is achieved, but the magnets on the rear side of the compressor wheel limit the design possibilities for the compressor wheel. Furthermore, also the mass moment of inertia of the compressor wheel is relatively large in this version, in order to achieve a significant increase in performance by switching energizing the electric motor.
The problem to be solved by the invention is that of producing an exhaust gas turbocharger for an internal combustion engine with an additional electric-motor drive, which provides for high efficiency and extensive design possibilities.
In an exhaust gas turbocharger for an internal combustion engine having an exhaust gas turbine arranged in an exhaust gas line, and a compressor arranged in an in-take duct and connected to the turbine by way of a shaft, the compressor includes a compressor wheel forming at the same time a rotor of an electric motor.
The rotor of the electric motor is an integral part of the compressor wheel, the rotor being contoured favorably for flow and thus also taking over the functions of the compressor wheel. As the rotor now no longer constitutes an additional component, but is integrally formed with the compressor wheel or is a part of the compressor wheel, the maximum possible degree of compactness is achieved.
The shape of the rotor is defined fundamentally by its function as compressor wheel. Both the. axial length and the radial extent of the compressor wheel can be adapted to flow-related requirements virtually without any limiting conditions, as can the shape and the design of the compressor wheel. At the same time, the dimensions of the compressor wheel can in principle be retained on account of the integration of the rotor into the wheel, so that the mass moment of inertia of the compressor wheel does not increase or does not increase significantly either.
A further advantage is that the electric-motor drive is separated from the lubricating oil of the exhaust gas turbocharger. The rotor is disposed on the air side of the compressor wheel, which does not come into contact with the lubrication system in order to keep the combustion air clean.
In a first embodiment, the entire compressor wheel consists of an electrically conductive material, so that the entire compressor wheel can be regarded as the rotor. This embodiment is characterized by a homogeneous material structure of the compressor wheel.
In a second embodiment, while the compressor wheel and the rotor consist of different materials but form a single component. In the arrangement the compressor wheel may consist of an electrically non-conductive material of lower density than the rotor, in order to reduce the mass moment of inertia. Also in this embodiment, the rotor is contoured favorably for flow and is to be regarded as a part of the compressor wheel as far as flow is concerned. This embodiment affords the advantage that conventional exhaust gas turbochargers can be retrofitted with a rotor contoured favorably for flow, in order to produce an additional electric-motor drive, and moreover, the flow behavior of the compressor wheel can also be positively influenced.
In a preferred embodiment, the compressor wheel is divided functionally into an auxiliary wheel and a main wheel, the auxiliary wheel forming the rotor, and the main wheel being arranged downstream of the auxiliary wheel in the direction of flow. With this functionally two-piece embodiment of the compressor wheel, additional design possibilities are obtained, via which the compressor behavior can be positively influenced. The auxiliary wheel and the main wheel can, for example, be provided with different free flow cross sectional areas, whereby the compressor characteristics can be influenced in a desired manner via the relationship between the flow cross sections of the auxiliary wheel and the main wheel. For example, the surge limit of the compressor can be changed positively by selecting the flow cross section in the auxiliary wheel so as to be smaller than the flow cross section in the main wheel. The throughput behavior of the compressor wheel as a whole can thus be influenced. On the other hand, if the relationship between the flow cross sections is reversed,xe2x80x94that is the flow cross section in the auxiliary wheel is greater than the flow cross section in the main wheelxe2x80x94the air flow can be accelerated in relation to the main wheel.
A further advantageous design possibility with regard to the aerodynamics of the compressor wheel as a whole lies in the selection of the number of blades in the auxiliary wheel and in the main wheel. It is possible, for example, to provide only half as many blades in the auxiliary wheel as in the main wheel, as a result of which in particular the weight and the mass moment of inertia of the auxiliary wheel can be reduced. In addition, the auxiliary wheel in this embodiment offers lower flow resistance.
The invention will become more readily apparent from the following description of different embodiments thereof described below on the basis of the accompanying drawings.